1. Field of the Invention
This invention relates generally to semiconductor devices, and more specifically to conductor deposition processes for semiconductors. In particular, this invention relates to reflow processes used to fill high-aspect ratio contacts (or vias).
2. Description of the Related Art
Due to increases in semiconductor packing densities, contact diameters have been reduced, thereby increasing height-to-width or aspect ratios. With increasing aspect ratios, adequate metal step coverage of contact surfaces has become more difficult to achieve, especially at temperatures lower than about 200.degree. C. As the aspect ratio increases, metal deposited at colder temperatures fails to produce good step coverage due to "necking" (or "cusping") at the top corners of contacts. Necking is detrimental because it gives rise to voids at the bottom of the contact, leading to reliability and yield problems. In an attempt to address this problem, various techniques have been developed and employed, including chemical vapor deposition of metals (CVD), laser reflow and aluminum reflow methods.
Of the methods used to fill contacts, the aluminum reflow method offers the advantages of lower cost and fewer process steps. However, most aluminum reflow processes proposed for filling high-aspect ratio contacts have not gained widespread acceptance due to the need for elevated reflow temperatures which are less desirable, particularly for second and higher metal levels. In these aluminum reflow processes, the temperature at which the metal layer is deposited is typically in the range of about 400.degree. C. to about 500.degree. C. These higher deposition temperatures may cause voiding and discontinuities in the metal layer due to the localized absence of sufficient metal to support continuous grain growth. As the aspect ratio increases, this problem may worsen because the amount of metal deposited onto the contact bottom and sidewalls decreases due to necking. High reflow temperatures may further increase the potential for voids by causing the early formation of widely spaced grains that lead to the formation of voids.
In an attempt to modify the aluminum reflow method for high aspect ratio use with lower reflow temperatures, aluminum alloy materials have been employed to reduce the melting point of the metal layer. For example, an Al--Ge--Cu damascene process using low temperature reflow sputtering has been used. In this process, Al--Ge--Cu alloy is deposited at room temperature onto the surface of contacts coated with Ti. The Al--Ge--Cu is then annealed at a reflow temperature of about 400.degree. C. The deposition and annealing steps are repeated as necessary to create a multi-level metallized interconnection. Although this method may be successfully used to create multi-level interconnections at lower reflow temperatures, it suffers from several problems. These problems may include difficulty in etching Al--Ge--Cu due to precipitation of Ge, and degradation in resistivity performance.
Although aluminum reflow processes are preferred due to low cost and simplicity, other techniques have been developed for filling high aspect ratio contacts. For example, flared or tiered contacts have been used to reduce the potential for necking at the contact corners. However, altering the geometry of contact corners results in the loss of semiconductor area. Low metal deposition rates have also been used to ensure adequate coverage, however, low deposition rates increase cost by limiting throughput.
Another method developed for filling high aspect ratio contacts utilizes a multi-step deposition process. In this process, a thin layer of metal is deposited at a cold temperature followed by the deposition of a thick layer of metal at a temperature of about 400.degree. C. to 500.degree. C. However, this process does not entirely eliminate the production of void spaces.
In a laser reflow process, a metal layer is deposited and then planarized by reflowing the metal with a laser. However, this process does not work well with aluminum or aluminum based alloys. When used to reflow aluminum, a native oxide forms on the aluminum and prevents planarization. Aluminum also requires a high optical pulse energy and variations in its surface topography can increase absorbed power and result in damage.
In another method, a metal layer is deposited in two steps using a partially ionized beam (PIB). In this method, a contact is first filled by a metal layer deposited at a temperature of about 150.degree. C. After the contact is filled, a second metal layer is deposited at a temperature of about 300.degree. C. This process produces adequate results, however, it is not practical for use in manufacturing applications due to a low deposition rate and a high substrate bias. The low deposition rate reduces throughput and increases the risk of gaseous inclusion into the metal layer. The high substrate bias is hard to maintain at a constant level and may damage semiconductor devices.
In yet another multi-step metallization process, a thick metal layer is deposited on a semiconductor wafer at a cold temperature of about 200.degree. C. or less. In a second step, the temperature is increased to approximately 400.degree. C. to 500.degree. C. while additional metal is being deposited. Although this method reduces the tendency for void formation, voids may still be formed if insufficient metal is deposited prior to increasing the temperature.
Still other techniques to improve the filling characteristics of aluminum have been tried. These methods include altering the surface characteristics of a contact by coating the contact with a material such as titanium or TiN to improve the wettability and coating conformance of aluminum. However, these methods suffer from reactions between aluminum and titanium that interfere with the reflow process, or require special treatment of a TiN layer, e.g. such as with a plasma treatment.
Consequently, a need exists for a practical and efficient method for filling high aspect ratio contacts. In particular, a need exists for a low cost reflow process that may be used to fill high aspect contacts in a void-free manner.